Mail attachment wrong media type Gmail API

EDIT

This first piece of code works for attachments with a combined size of a few mb. If you want to use the allowed limit of 35 mb, check the edit at the end of the answer.

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After Steve pushed me in the right direction (the entire mail has to be in the “raw”-parameter), I simply tried the Python API and looked at the mail generated by that.

Mail without attachment

Content-Type: text/plain; charset="UTF-8"
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
to: [email protected]
from: [email protected]
subject: Subject Text

The actual message text goes here

Mail with attachment

Content-Type: multipart/mixed; boundary="foo_bar_baz"
MIME-Version: 1.0
to: [email protected]
from: [email protected]
subject: Subject Text

--foo_bar_baz
Content-Type: text/plain; charset="UTF-8"
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit

The actual message text goes here

--foo_bar_baz
Content-Type: image/jpeg
MIME-Version: 1.0
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="example.jpg"

{JPEG data}

--foo_bar_baz--

So I just wrote my code around this, and it worked great!

var reader = new FileReader();
reader.readAsDataURL(attachment);
reader.onloadend = function (e) {
  // The relevant base64-encoding comes after "base64,"
  var jpegData = e.target.result.split('base64,')[1];
  var mail = [
    'Content-Type: multipart/mixed; boundary="foo_bar_baz"\r\n',
    'MIME-Version: 1.0\r\n',
    'to: [email protected]\r\n',
    'from: [email protected]\r\n',
    'subject: Subject Text\r\n\r\n',

    '--foo_bar_baz\r\n',
    'Content-Type: text/plain; charset="UTF-8"\r\n',
    'MIME-Version: 1.0\r\n',
    'Content-Transfer-Encoding: 7bit\r\n\r\n',

    'The actual message text goes here\r\n\r\n',

    '--foo_bar_baz\r\n',
    'Content-Type: image/jpeg\r\n',
    'MIME-Version: 1.0\r\n',
    'Content-Transfer-Encoding: base64\r\n',
    'Content-Disposition: attachment; filename="example.jpg"\r\n\r\n',

    jpegData, '\r\n\r\n',

    '--foo_bar_baz--'
  ].join('');

  // The Gmail API requires url safe Base64 
  // (replace '+' with '-', replace "https://stackoverflow.com/" with '_', remove trailing '=')
  mail = btoa(mail).replace(/\+/g, '-').replace(/\//g, '_').replace(/=+$/, '');

  $.ajax({
    type: "POST",
    url: "https://www.googleapis.com/gmail/v1/users/me/messages/send",
    headers: {
      'Authorization': 'Bearer ' + accessToken,
      'Content-Type': 'application/json'
    },
    data: JSON.stringify({
      raw: mail
    })
  });
}

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Edit

The code above works, but a few alterations are needed to use the max limit of 35 mb.

With a mail built up as the example under the heading Mail with attachment, the altered ajax-request looks as follows:

$.ajax({
  type: "POST",
  url: "https://www.googleapis.com/gmail/v1/users/me/messages/send?uploadType=multipart",
  headers: {
    'Authorization': 'Bearer ' + accessToken,
    'Content-Type': 'message/rfc822'
  },
  data: mail
}); 

The Content-Type is now message/rfc822 instead of application/json, the url has gotten a new parameter uploadType=multipart, and most importantly the mail is no longer Base64 encoded, but supplied in the rfc822-format.